Get reference to a void* element of the internal extensible array.
  Returns a reference to the void* element correponding to the index idx of the internal extensible array. If idx is an index to a new element and the internal extensible array is not long enough, the function extends it with zero-initialized elements as necessary to include it.
  idx should be a value previously returned by xalloc.
  The refence returned may become invalid after another call to this member with a different value for idx, after a call to member ios::copyfmt or when the object is destroyed.

  The internal extensible array is a general-purpose array of word elements.

Parameters.

idx

The index value of the internal extensible array which reference value is requested.

Return Value.
  A reference the element which index is idx in the internal extensible array.
  This value is returned as a reference to a void* object.

See also.
  xalloc, iword
  ios_base class